Is split-phase AC the same as "regular" AC?

[Siwastaja]

There are only 2 high-voltage lines feeding the transformer on a pole behind where I live, so I'm assuming there must be only 2 "phases" (split phase).

You are getting closer to grokking it! You just applied "2 phases" to 2 wires, which, of course, can only carry one signal. It's the same signal, depending on which of the two wires you take as a reference. Center-tapped transformer scales the amplitude, and adds a reference in the middle.

If you take the voltage between any two wires in 3-phase system (your upstream power distribution eventually) - any two out of the 3 or 4 wires -, you get a single phase, not 2-phase power. 1 phase is 2 wires. 2 phase-power is 3 wires, with double the current flowing in neutral, which is why 3 phase power was invented, which still is 3 wires (optionally, fourth).

High-voltage (high power) distribution is 3-phase everywhere. Yet, a fridge or bulb runs on a single phase everywhere. If you compare US/UK vs. Europe, the point where the single phase branches off the 3-phase distribution, is different. In US/UK whole houses get a large single-phase supply, say 100-200A 240V (US splits that with the center tap, too, which is another orthogonal design choice). In Europe it's more typical that houses get smaller-current 3-phase supplies (say 3 x 35A); apartments have 1-phase supply.

[madires]

So.  Transmission lines in the US are not 3 phase?  Or do those little dustbin sized transformers you all seem to put on poles converting out two 180* phases from the 3?

In the UK the 11kV line is dropped down by minivan sized transformer like 2m cubed.  hidden in slack land where houses don't fit. That produces spurs of 3 phase which winds it's way under the roads.  One 240V phase brought out and drops a feed into a sequence of houses, another phase does a different sequence of houses and at the end of the cable run there is roughly the same number of house on each phase.

AFAIK, in the US the local distribution is often faned out differently, i.e. one phase/wire (of three, typically 7.2 kV) runs down the poles along a street feeding pole mounted transformers which provide the split-phase to homes. Load distribution across the three phases is done via streets. Based on the number of customers/buildings the poles can carry also two phases or all three. In more densely populated areas you can find pad mounted transformers.

[paul cotter]

We still seem to have some confusion here. I strongly recommend the diagrams on post #50, kindly supplied by pdenisowski which explains the US system better than any concoction of words.
PS: That diagram refers to a four wire system which is the system used in Ireland in urban areas and universally used in the UK. In rural areas in Ireland a three wire system is used in which case the transformer primary is fed from two of the phases, ie no neutral wire.

[bdunham7]

So.  Transmission lines in the US are not 3 phase?

Generation and HV distribution lines are 3-phase.  Where the HV distribution goes into what we call a "substation" that HV is stepped down to the medium voltage that is distributed to buildings.  This could be 4160V to 13800V or even higher or lower voltages in some cases.  In rural and residential areas these medium voltage distribution lines often go a long way (>50 miles in some instances) as only a single phase.  Older installations even had one-wire ground-return systems.  In the case of my suburban residential area, the main backbone of this distribution system is a 4-wire wye with 13.8KV phase-to-neutral.  This is tapped at various points to send one phase at 13.8kV down the various streets and alleys where the pole-pig transformers will step it down to the 240V split-phase that we're talking about.

[TimFox]

In my residential area of Chicago, the pole transformers in the alley behind my house (one per block) are fed by 3000 V (single phase wrt ground) and feed the houses with 240 V split phase.  Commonwealth Edison is slowly upgrading the system to 6000 V.  At my former employer (in a modern suburban office park), each building had its own transformer (two in my case) fed by high-voltage three phase.  Edison supplied the transformers to order for 208 V, 440 V, etc. (line-line) for the customers.

[IanB]

So.  Transmission lines in the US are not 3 phase?  Or do those little dustbin sized transformers you all seem to put on poles converting out two 180* phases from the 3?

Yes, transmission and distribution is three phase. But the cylindrical "pole pig" transformers you see on poles are single phase transformers, with a single primary winding and a centre-tapped secondary winding of 120-0-120 V. The primary is connected between one of the distribution phases and ground/neutral, with the transformer can being grounded.

A typical primary voltage is 12.47 kV phase to phase, or 7.2 kV phase to ground.

In the UK the 11kV line is dropped down by minivan sized transformer like 2m cubed.  hidden in slack land where houses don't fit. That produces spurs of 3 phase which winds it's way under the roads.  One 240V phase brought out and drops a feed into a sequence of houses, another phase does a different sequence of houses and at the end of the cable run there is roughly the same number of house on each phase.

What you are describing happens in towns and urban housing estates.

In rural villages you may still find in the UK something similar to the USA, with transformers sitting on poles, and the 240 V supply wires strung along poles and dropped through the air to the houses. Although there has been a tendency to move this infrastructure underground over time.

[IanB]

In the case of my suburban residential area, the main backbone of this distribution system is a 4-wire wye with 13.8KV phase-to-neutral.  This is tapped at various points to send one phase at 13.8kV down the various streets and alleys where the pole-pig transformers will step it down to the 240V split-phase that we're talking about.

The distribution voltage is most typically measured phase to phase, so a typical 13.8 kV system would be 13.8 kV phase to phase and 8 kV phase to ground.

A classic pole pig has a single primary bushing on top to receive one of the incoming phases, and it drops this to ground through the transformer casing.

Across North America, the most common distribution voltage is 12.47 kV phase to phase and 7.2 kV phase to ground.

[Monkeh]

There are only 2 high-voltage lines feeding the transformer on a pole behind where I live, so I'm assuming there must be only 2 "phases" (split phase).

There may be one or two phases (if both wires are on insulators, there are two, often there's only one and the return is ground), they're from a 3-phase system and are 120° apart. You can't tell that without a third reference point (the third phase or ground).

I seem to recall we've seen yours previously: https://www.eevblog.com/forum/chat/(n-american)-line-voltage-120-110-117/msg5852779/#msg5852779

This is single-phase using the ground conductor as a return, as far as I can see. I don't see an insulator on the second wire on the pole, I see no second bush on the transformer, and there's only a single HV fuse.

The utility company must have two completely separate power-distribution networks, one split-phase, the other 3-phase.

They do not. They simply tap off one or two phases to feed single phase loads.

[mawyatt]

A classic pole pig has a single primary bushing on top to receive one of the incoming phases, and it drops this to ground through the transformer casing.

That seems dangerous using case as the return!! If the case becomes disconnected from ground, the transformer case rises the hot side potential!!

Best

[IanB]

I'm no expert in power transmission, so I may be mistaken about part of this.
Some transmission lines here are 3 phase, others are not (just split-phase).
There are only 2 high-voltage lines feeding the transformer on a pole behind where I live, so I'm assuming there must be only 2 "phases" (split phase).

No, "split phase" is where you have a center-tapped transformer (two hots and a neutral) on the low voltage side. On the distribution side it is a three phase system. There is no split phase there.

Of course, industrial feeds are all 3-phase. Not sure where the division is between the two; seems like all residential lines are split-phase here.

No, two wires does not mean split phase.

The utility company must have two completely separate power-distribution networks, one split-phase, the other 3-phase.

No, it's all three phase.

I don't see how you could derive 2 180°-apart phases from a 3-phase line (0°-120°-240°), which I think is what you were getting at.

Take any two wires from a three phase supply and you get a single phase. You could take a pair of phase wires and get 12.47 kV phase to phase, or you could take one phase wire to ground and get 7.2 kV phase to ground.

The most common arrangement is that each pole pig takes one distribution phase to ground.

[Monkeh]

A classic pole pig has a single primary bushing on top to receive one of the incoming phases, and it drops this to ground through the transformer casing.

That seems dangerous using case as the return!! If the case becomes disconnected from ground, the transformer case rises the hot side potential!!

Best

Typically these will have multiple bonds.

[IanB]

A classic pole pig has a single primary bushing on top to receive one of the incoming phases, and it drops this to ground through the transformer casing.

That seems dangerous using case as the return!! If the case becomes disconnected from ground, the transformer case rises the hot side potential!!

Best

Maybe so, but that arrangement is rather common. Great care is taken to securely ground all transformer casings, whatever the primary side arrangement. The transformer casing is also the neutral on the secondary side, and this must be grounded to maintain a safe neutral.

[Randy222]

Split phase is just a single phase bipolar or normal/inverted polarity phase.  Think of it like unbalanced vs balanced audio.  It's the same signal.  As others have said it's just a centre-tapped 240V system.  rather than having 1 x 240V signal and 1 x Neutral signal you have 2 x 120V signals out of phase.

1ph is 1ph. No shift, no inversion.

Some see a flip because they flip the probes over (using C.tap as the gnd point).

Use one or two o-scopes and probes on a low voltage C.tap xfrmr (12vac w/ C.tap).

(isolated secondary w/ isolated scope inputs)
Probe1------GND
S1-----------C.tap----------S2
                Probe2---------GND


Voltage only builds or reduces in one direction (vector) at any given time. Why? Because there's only one mag field vector.

When L1 reaches +6v, L2 is becomes -6v. end-end is 12v differential, end to tap is 6v.

People get tricked when they make C.tap the common for both probes, which make the 1ph look like inversion or 180 shift, it's not.

This setup(view) is deceiving, but is accurate in physics realm.
Probe1------GND
L1-----------C.tap----------L2
                  GND---------Probe2

Look at it carefully, notice the physical probe is physically flipped over 180deg.

For simple coil xfrmers, unless a mag field cuts coils (or tapped sections of one coil) at different times, then there can only be one phase. You also do not see 3ph on one wire, you must have three 1ph wires.

[Randy222]

Yes, transmission and distribution is three phase. But the cylindrical "pole pig" transformers you see on poles are single phase transformers, with a single primary winding and a centre-tapped secondary winding of 120-0-120 V. The primary is connected between one of the distribution phases and ground/neutral, with the transformer can being grounded.

It would be more accurately stated like +120v --- c.tap --- -120v  (VAC rms).
It's a nice sine wave, but from physics it's how the electrons are pushed from one side to the other. As one side goes neg the other side goes pos, charge conservation end-to-end, thus +120 + -120 =0, but the differential (+120 - -120)=240.

[studiot]

So.  Transmission lines in the US are not 3 phase?  Or do those little dustbin sized transformers you all seem to put on poles converting out two 180* phases from the 3?

Yes, transmission and distribution is three phase. But the cylindrical "pole pig" transformers you see on poles are single phase transformers, with a single primary winding and a centre-tapped secondary winding of 120-0-120 V. The primary is connected between one of the distribution phases and ground/neutral, with the transformer can being grounded.

A typical primary voltage is 12.47 kV phase to phase, or 7.2 kV phase to ground.

In the UK the 11kV line is dropped down by minivan sized transformer like 2m cubed.  hidden in slack land where houses don't fit. That produces spurs of 3 phase which winds it's way under the roads.  One 240V phase brought out and drops a feed into a sequence of houses, another phase does a different sequence of houses and at the end of the cable run there is roughly the same number of house on each phase.

What you are describing happens in towns and urban housing estates.

In rural villages you may still find in the UK something similar to the USA, with transformers sitting on poles, and the 240 V supply wires strung along poles and dropped through the air to the houses. Although there has been a tendency to move this infrastructure underground over time.

Plenty of urban areas in the UK still have electricity supplies (and transformers) on poles.

It is just too costly to 'underground' still functional older installations.


There are, of course, arguments for and against undergrounding v aerial supplies.
Our local hospital has an answer I would like to see established whereby the underground services are carried not in backfilled trenches but in concrete channels with sectional concrete lids.
Not only that but each service has a designated place.

[themadhippy]

whereby the underground services are carried not in backfilled trenches but in concrete channels with sectional concrete lids.

As was done along the regents canal to bring HV  cables  into central london.Some sources also claim theres a double bonus as the canal water is used to keep the cables cool

[gf]

It would be more accurately stated like +120v --- c.tap --- -120v  (VAC rms).

Conceptually it amounts to the right thing, but the notation is a bit flawed. RMS values are strictly magnitudes and cannot be negative. The opposite signs belong to the instantaneous V(t) waveforms (which are 180° out of phase), not to the RMS values.

[Randy222]

It would be more accurately stated like +120v --- c.tap --- -120v  (VAC rms).

Conceptually it amounts to the right thing, but the notation is a bit flawed. RMS values are strictly magnitudes and cannot be negative. The opposite signs belong to the instantaneous V(t) waveforms (which are 180° out of phase), not to the RMS values.

Physics is always right, notation can be utterly a royal pita. Engineers usually label things.

[IanB]

Physics is always right, notation can be utterly a royal pita. Engineers usually label things.

Indeed. But AC is never labeled with +/−. If we write 120-0-120 V AC, then the center zero makes the phasing clear, and no further labeling is needed.

[Analog Kid]

So just for my own benefit, is it safe to say that this is how split phase is derived from 2 phases of 3-phase power?



(I only drew 3 lines for the 3-phase part. There's no ground conductor there, is there?)

[IanB]

So just for my own benefit, is it safe to say that this is how split phase is derived from 2 phases of 3-phase power?

(Attachment Link)

(I only drew 3 lines for the 3-phase part. There's no ground conductor there, is there?)

Basically, yes, but the primary (MV) side is an uncommon arrangement, as it requires two line conductors to feed the transformer.

It is more common to ground the low side of the MV primary winding and common it with the LV ground/neutral. (The three MV phases will commonly come from a grounded wye secondary of the supply transformer at the substation, so ground/neutral can be the return from the pole pig back to the substation.)

This arrangement saves money on the pole transformer (it only needs one primary bushing), and on the local distribution network (you only need to take one MV phase around the houses to feed the pole transformers).

You can see this nicely illustrated in this video:



In the thumbnail image, you can clearly see the single primary bushing receiving the 7.2 kV supply, and the center-tapped 120-0-120 V secondary bushings, with red and black wires coming from the hot terminals.

[Analog Kid]

So just for my own benefit, is it safe to say that this is how split phase is derived from 2 phases of 3-phase power?

(Attachment Link)

(I only drew 3 lines for the 3-phase part. There's no ground conductor there, is there?)

Basically, yes, but the primary (MV) side is an uncommon arrangement, as it requires two line conductors to feed the transformer.

It is more common to ground the low side of the MV primary winding and common it with the LV ground/neutral. (The three MV phases will commonly come from a grounded wye secondary of the supply transformer at the substation, so ground/neutral can be the return from the pole pig back to the substation.)

Something like this?

[Randy222]

So just for my own benefit, is it safe to say that this is how split phase is derived from 2 phases of 3-phase power?

(Attachment Link)

(I only drew 3 lines for the 3-phase part. There's no ground conductor there, is there?)

IanB is more detailed. Analog Kid show how it's normally done in Reply #97.

Notice in your pic reply #95, you did not "split" any phase on the primary side. You attached the primary to two phases that are out of phase with each other by 120deg. Only the secondary winding in your pic is "split", or more precisely Center Tapped.

red being the resultant between your primary L1 L2.



Two actual phases 180 shifted from each other


Two phases (coils) cut at the exact same time, aka "in-phase"

[IanB]

IanB is more detailed. Analog Kid show how it's normally done.

Notice in your pic, you did not "split" any phase on the primary side. You attached the primary to two phases that are out of phase with each other by 120deg. Only the secondary winding in your pic is "split", or more precisely Center Tapped.

red being the resultant between your primary L1 L2.



Two actual phases 180 shifted from each other

This is completely confusing. What are you even trying to say here?

In both of Analog Kid's diagrams, there is only one phase feeding the primary of the transformer, and only one phase on the secondary. So there are no phase angles, no 120° anywhere.

[IanB]

So just for my own benefit, is it safe to say that this is how split phase is derived from 2 phases of 3-phase power?

(Attachment Link)

(I only drew 3 lines for the 3-phase part. There's no ground conductor there, is there?)

Basically, yes, but the primary (MV) side is an uncommon arrangement, as it requires two line conductors to feed the transformer.

It is more common to ground the low side of the MV primary winding and common it with the LV ground/neutral. (The three MV phases will commonly come from a grounded wye secondary of the supply transformer at the substation, so ground/neutral can be the return from the pole pig back to the substation.)

Something like this?

(Attachment Link)

Since Randy222 is trying to confuse you, I will respond with a simple: Yes. Basically, that is a common arrangement.

You could even cross out the two unused phases of the three phase supply. If you watch the video I linked, you will see there is only a single phase carried along the top of the distribution poles. Just one wire. Sometimes they don't even bother to carry more than one phase around a local distribution circuit.

There will, however, usually be a neutral return wire back to the substation where you have drawn the ground symbol. You occasionally find "single wire, earth return" circuits, but it is rare as the ground has a higher impedance than a return wire, and it is less efficient to do it that way.